Per-role cooldown system for channels

Problem

Right now we can only set a single constant cooldown per-channel, being unable to implement a chatting-rapidness optimization based on trust, so newly joined members can either send messages as rapid as the oldest member or can't send at all. (Excluding that we can disable slowmode for a role, since it's not a practical way to optimize the chat as explained, and except the fact that we can limit what they can send.)

Proposed solution

Bring per-role cooldown system for channels!!!!!!! In channel settings -> roles&permissions, add an option to set a cooldown value, so we can set a cooldown value per role! This way community leaders can limit newly joined members powerfully, using important role of cooldown logic.

Notes (optional)

Leave the current way for it as it is, let it represent the default cooldown